Can't edit particle fluid or boid physics settings through python
Thanks for the clear question. I just added the bpy.types.Scene.update call and in Blender 2.79a it works.

import bpy

bpy.ops.mesh.primitive_plane_add()
emitter = bpy.context.object

mod = emitter.modifiers.new("ParticleSystem", type = 'PARTICLE_SYSTEM')
stngs = mod.particle_system.settings

stngs.physics_type = 'FLUID'
bpy.context.scene.update()
stngs.fluid.solver = 'CLASSICAL'    #This line results in the error
stngs.fluid.fluid_radius = 0.05     #So does this one if I skip the one above


Sleep

sleep will never work in Blender as blender will halt mostlyduring the running of a script. Blender will just wait until the script has finished completely and then run the internal variant of bpy.types.Scene.update.